For many centuries, mankind has been practicing the distillation process of obtaining purified water from dirty water. This process involves boiling the dirty water to create steam and condensing the steam back into water. As the contamination of sources of drinking water by industrial wastes and other discharges has become ever more serious in many localities, there has been an increasing demand for an economical and reliable method of obtaining purified water from contaminated water. Modern technology has responded to this demand by developing better filtration technology, such as ultra and reverse osmosis membrane filters; adsorption technology, such as activated carbon columns; and ion exchange technology of various synthetic resins. Although these methods of water purification have been widely accepted and implemented in many industries as well as private dwellings, the cost of installation and operation of such systems is often too high for many average home owners. As an alternative method, the age-old practice of obtaining purified water through distillation by using heat from the sun has been upgraded and improved recently, and is receiving new attention in many under-developed countries. Without exception, the present-day practice of solar distillation employs the so-called solar pond with a transparent cover, allowing solar radiation on the shallow water in the solar pond to heat the water and create evaporation of the water into steam. The steam in contact with the transparent cover loses heat to the atmosphere through the cover and condenses into water drops, which travel down the inside surface of the cover and collect in gutters. As the evaporation of the water in the solar pond takes place only on the free surface of the water, a large free surface area is required in order to evaporate a sizable amount of the water. For this reason, the solar pond requires a shallow pond with a large area.
In view of the fact that the major portion of the heat released by the condensing steam is not recycled since it is lost to the atmosphere through the transparent cover, the solar pond is a primitive system with a low working efficiency. Of course, the sunlight is free, and consequently, there is little requirement for high efficiency in utilizing the sunlight itself. However, the low efficiency of the solar pond creates a problem since the requirement of a large pond area is simply not compatible in many localities with a heavy population density, and a pond with a large area requires more materials of corrosion resisting of grade for construction. A better solar still can be provided by recycling the heat released by the condensing steam to preheat the water to be distilled. This provides an advantage in that the total surface area collecting the sunlight required to evaporate a given amount of water per day can be made significantly smaller compared to the solar pond, resulting in the saving of the space and materials required to construct a solar still. A better solar still can be provided by minimizing the wetted area in the still with a given area for collecting sunlight. The wetted area in a solar still can be reduced for a given amount of the distilled water required per day if the water can be evaporated not only on the free surface but also within the volume of the water--the water has to be boiled. It is a well known fact that the water boils when it is heated or the ambient pressure is lowered. In a better still, the water should be heated by solar energy and, at the same time, the pressure in contact with the water should be partially evacuated in order to promote the boiling of the water.
The primary object of the present invention is to provide a solar still wherein the evaporation of the water heated by the sunlight takes place in the partially evacuated tubings.
Another object of the present invention is to use the vacuum legs on the condensate discharge and the reject discharge in order to maintain the partial vacuum in the heater tubing wherein evaporation of the water takes place.
A further object of the present invention is to provide a solar still including a heat exchanger-condenser that condenses the steam into the water and preheats the water being fed into the heater tubings heated by the solar collector.
Still another object of the present invention is to employ a flow regulating orifice to drip-feed the water into the heater tubings from the feed reservoir in order to minimize the reject flow out of the heater tubings which are wetted with feed water on the bottom half or less of the cross section.